This invention relates to a signal amplitude and phase detection systems and, in particular, to systems for determining the phase of a periodic signal with reference to the point at which it is at 50% of its maximum amplitude.
In signal processing and measurement systems it is often necessary to identify a particular point in the waveform of a periodic signal and to synchronize the phase of the output of an oscillator based upon that point. For example, in measuring the characteristics of a television waveform using a vector-scope to compare the amplitude and phase of one component of a video signal with the amplitude and phase of another, it is desirable to generate a reference signal locked in phase with the 50% amplitude point of the horizontal sync pulse of that video signal. This is typically accomplished by employing a "sync stripper" circuit, that is, one which produces a signal representative of the horizontal sync pulse only, followed by a circuit, such as a phase locked loop, which generates a signal locked in phase with the output of the sync stripper.
One example of the aforementioned approach is employed in the Leitch Model 730-N SC/H phase meter wherein a subcarrier signal is locked to the phase of the horizontal sync pulse. The maximum excursion of the sync pulse ("sync tip") is clamped to a known value, the sync pulse is assumed to have a predetermined amplitude, and the 50% point of sync is detected by comparing the sync signal to a level equal to 50% of that predetermined amplitude. This approach to 50% amplitude detection has the drawbacks that it is sensitive to actual signal amplitude and temperature. Insofar as it is used to establish the phase of a video signal it has the drawback that it requires a pulse generation stage to discriminate between horizontal sync and vertical interval pulses, which increases complexity and temperature sensitivity.
Another example of a sync stripper is employed in the Model 3258 SC/H phase meter manufactured by the Grass Valley Group of Tektronix, Inc. In this case, one sample and hold circuit determines the voltage of sync tip and another determines the voltage of the "back porch" of a first sync component of the video signal. A resistive divider establishes the 50% amplitude value, which is compared to the video signal to detect the 50% point on the next received sync pulse waveform. This approach also has the drawbacks of being sensitive to variations in the amplitude of the sync pulse and to temperature.
In a system for the detection of the 50% amplitude point of a waveform and synchronizing a signal to that point, it is desirable to employ circuitry that precisely recognizes that point with minimal complexity. Gated phase detector and phase locked loop circuitry which exhibits such characteristics is disclosed in Crosby U.S. Pat. No. 4,316,150, issued Feb. 16, 1982.
Therefore, it would be desirable to provide an improved 50% point of amplitude and phase detector system which is not dependent on a fixed signal amplitude and which exhibits improved temperature stability. In application to a video signal, such a circuit should also be accurate despite some leading edge distortion in the shape of the horizontal sync pulse. It would be particularly desirable to employ, in combination with the circuitry disclosed in the aforementioned Crosby patent, additional circuitry that would take advantage of its desirable characteristics.